The present invention relates to the protection of turbine shafts against excessive axial thrust loads, particularly during emergency shutdown or trip conditions.
The interior of a turbine is composed of a plurality of chambers which may be separated from one another by groups of rotor blades, rotor discs, and seals of various types, such as rotor labyrinth seals which have stepped diameters. The pressures in these chambers have varying values, due, for example, to pressure and fluid flow velocity differences across rotor blades and pressure differentials across rotor discs and rotor seals. Each of these differentials may act in one axial direction or the other, and their sum determines the net axial thrust imposed on the rotor shaft.
Turbines are normally designed so that the axial pressure load acting on the rotor is essentially balanced, this being achieved primarily by proper choice of labyrinth seal diameters. Any residual axial thrust loads are supported by a thrust bearing. In general, steps must be taken to assure that the axial rotor thrust remains below a given level because an excessive axial thrust can overload the thrust bearing and lead to serious machine damage.
Since the pressures in the various chambers vary over the turbine load range, limitations imposed by the rotor geometry of certain turbines can create an obstacle to maintenance of an acceptable net axial thrust.
It has been found that, for many types of turbines, this problem can be alleviated by redistributing the pressures acting on the rotor. This may be done, for example, by interconnecting certain chambers, or cylinder pressure zones, by means of equilibrium pipes containing flow restricting orifices. Appropriate sizing of such orifices can then produce a pressure distribution suitable to maintain a low steady state net rotor thrust.
However, this solution has been found to be effective only when the turbine is in normal operation and if a turbine must be rapidly shut down, as in the case of an emergency shutdown, the pressure distribution established during normal operation can be substantially altered and can result in an excessive axial load on the thrust bearing.